North Korea has begun evacuating some citizens into tunnels with emergency provisions and putting military camouflage on buses and trucks, the South Korean Defense Ministry said Tuesday.

North Korea has dramatically increased the number of fighter jets in its skies over the last few days, an unnamed government official told South Korean-based news agency Yonhap, and may have closed access to its limited internet from the outside.

“Flights of North Korean air force’s fighter jets and helicopters reached about 700 sorties on March 11,” the source said. South Korean sources often only reveal information to local media on the condition of anonymity.

All that was left now, was for North Korea to “unleash merciless retaliation” against ongoing U.S.-lead sanctions and war games, Yonhap reported, citing an official North Korean government statement.